One prior art of such radar responder is shown in Japanese Patent Publication No. Sho. 55-31430.
In the radar responder (radar indicator) shown in said Japanese Patent Publication, when the radar responder held by a survivor at sea receives a radar pulse from a searcher's radar, the radar responder operates to transmit a radar responding signal which sweeps a predetermined frequency range with a predetermined period, responding thereto. When the searcher's radar receives the radar responding signal, continuous dotted lines with predetermined intervals therebetween are displayed on a radar scope which distinguish from the reflected signals from other radar reflectors so as to indicate the position of the survivor.
In such devices, it is only necessary to generate radar responding signals which have weak electric field intensity, and this can be accomplished by small and cheap devices. This also provides a wide range of use such as for life boats in large ships, for small ships, and for leisure boats. To be used commonly in such a wide range, it is an important object to prevent unnecessary and erroneous transmissions, thereby obtaining an accurate operation.